The present invention relates to a charged particle beam detection system of charged particle beam apparatus and a charged particle beam apparatus equipped with the charged particle beam detection system and more particularly, to a scanning electron microscope (SEM) for forming an image of a signal of electrons such as secondary electrons generated from a specimen while scanning an electron beam. More specifically, the present invention is concerned with method and system for inspecting a substrate having minute circuit patterns such as a semiconductor device or liquid crystal device and particularly, concerned with a review SEM technology of performing pattern inspection and defect review of the semiconductor device and a photo-mask as well.
As generally known, in the SEM for scanning an electron beam on a specimen to form an image of a signal of electrons such as secondary electrons generated from the specimen, the contrast of an acquired image depends on conditions of landing or irradiation of the electron beam on the specimen, for example, irradiation beam current amounts, landing energy levels and potential distribution on the specimen and also, largely depends on the scheme (type) and shape of a detection system. The image contrast is known as including voltage contrast reflecting a charging distribution of the specimen, shadow contrast reflecting topography or unevenness of the specimen surface and substance contrast reflecting differences in the kind of substance of the specimen.
When using the SEM for evaluation/inspection of semiconductor devices, a technology for highly sensitively detecting a delicate uneven portion on the semiconductor device surface to provide a shadow contrast image, among various kinds of image contrast as mentioned above, is thought much of more and more.
A semiconductor device is fabricated by repeating a step of transcribing, through the lithography process and etching process, a pattern formed on a wafer by using a photo-mask. In the fabrication process as above, for the purpose of realizing early start-up of yield and stable running of the fabrication process, it is indispensable to speedily analyze defects found through an in-line wafer inspection and make use of the result of analysis for countermeasures. The key to speedily connecting the result of inspection to the trouble shooting is involved in an automatic defect review/classifying technique for reviewing many detected defects at a high speed and sorting the defects in accordance with causes of generation. As the minuteness of fabrication process advances, the defect size affecting the yield of semiconductor production has also been made to be minute and with an optical type review apparatus, reviewing a defect at high resolution is difficult to achieve. Under the circumstances, a review apparatus of SEM type capable of performing review at high resolution has been put into production. In order to detect a minute foreign matter present on the device and unevenness caused by, for example, scratching as well with this type of apparatus, it is important to obtain a shadow image based on an SEM image equivalent to a shade caused when light is irradiated obliquely. The basic principle of acquisition of the shadow image as above will be described below.
A description will be given by using an example as shown in FIG. 2A in which an uneven portion 101 caused by the presence of a foreign matter in a film is scanned with an electron beam 37 as shown at arrow 41 to irradiate the right side of the uneven portion 101 with the electron beam. Under the irradiation of the electron beam, secondary electrons 38 are emitted. When noticing components of low elevation angles at that time, secondary electrons emitted to the left side are partly shielded by the uneven portion 101. Consequently, the number of secondary electrons detected by a left detector 11 differs from that detected by a right detector 12. Images obtained with the detectors 11 and 12 in this manner provide images emphasized in shadow as shown in FIGS. 2B and 2C, respectively. Therefore, by arranging the detectors on the right and left sides of the primary electron beam 37, the image contrast reflecting the unevenness of the specimen can be obtained.
The principle of obtaining the shadow contrast of the surface by separating secondary electron signals into two directions of right and left is described in, for example, “Electron signal and detector strategy in electron beam interactions with solids” by L. Reimer, published by D. F Kyser et al (SEM Inc., AMF O'Hare IL, P. 299, 1982). In this reference, secondary electrons are not fetched directly by a detector but are caused to impinge upon the bottom surface of an objective lens and electrons generated from the lens bottom surface are separated into tow directions so as to be detected.
Energy distribution of secondary electrons is illustrated graphically in FIG. 3. As well known in the art, secondary electrons are distributed in relation to energy such that the number of components generated at energy levels of about several of eV is the largest, reducing to the number of components at about 50 eV. Also, as shown in the graph, electrons having substantially the same energy as landing energy of the primary electron beam are generated and they are termed so-called reflected electrons or backscattering electrons. The expression of secondary electrons herein implicitly indicates components of backscattering electrons having landing energy approximating that of the primary electron beam. This holds true in the following description.
The technique of acquiring the shadow contrast by separating secondary electrons into right and left components while keeping the outgoing or emitting angle from the specimen of secondary electrons unchanged utilizes the fact that secondary electrons are emitted from a specimen by having a peculiar distribution in the elevation angle direction and a peculiar distribution in the azimuth angle direction in accordance with the unevenness of the specimen and therefore, it is very important for making the shadow highly contrasted that detection is carried out without disturbing the directional distribution of secondary electrons at the time of their emission from the specimen. Conversely, if a process of changing the directional distribution of secondary electrons prevails between the specimen and each of the detectors, the shadow contrast is lowered, failing to discriminate between shadows. Considered as a member affecting the directional distribution of secondary electrons is an objective lens, for instance. As the secondary electrons pass through a magnetic field of the objective lens, the trajectory of secondary electrons undergoes rotational actions in accordance with energy levels of the secondary electrons, so that the directional distribution of the secondary electrons on the specimen is disturbed. Accordingly, for the sake of acquiring the shadow contrast, the right and left detectors are arranged in general under the objective lens, that is, between the specimen and the objective lens as in the aforementioned reference of Reimer.
On the other hand, an SEM especially used in recent years for reviewing defects in semiconductor patterns is generally required to maintain high resolution at a low landing energy level of about 1 keV and to meet this requirement, the construction of components above the specimen, too, is designed optimally. The shorter the distance between the principal plane of a magnetic field of the objective lens and the specimen (called working distance), the higher the resolution can become and therefore, a design is so made as to shorten the distance between the magnetic field center and the specimen. In addition, with the aim of landing the primary beam at a low energy level of about 1 keV while maintaining the high resolution, negative potential (called retarding voltage) is applied to the specimen so that the primary beam may be decelerated for irradiation immediately before the specimen or an objective lens is used in which an electrode at positive potential (called a boosting electrode) effective to permit the beam to be highly accelerated for passage through the objective lens is inserted to thereby superimpose an electromagnetic field and attain the high resolution performance. This kind of lens are also called as electric immersion lens.
As the principal plane of the objective lens approaches the specimen, secondary electrons are involved in the magnetic field of objective lens above the specimen and undergo a rotational action. A known example of achieving the high resolution performance while detecting minute unevenness, backgrounded by the aforementioned principle, is disclosed in JP-A-8-273569, for instance. For attaining the high resolution, an electric immersion lens effective for high resolution is used. Secondary electrons generated from a specimen at low energy levels are rotated by a lens magnetic field through rotation angles according to the energy levels. As a result, after the secondary electrons have passed through the objective lens, their directional information is lost. To cope with this problem, an electric field for accelerating secondary electrons is generated near a wafer to enable the secondary electrons to pass at high speeds through the magnetic field generated by the objective lens, thereby ensuring that the difference in rotation angle dependent on energy can be reduced and the directional information can be preserved. Further, by controlling trajectories of secondary electrons at low energy and backscattering electrons, the backscattering electrons can be detected in an inner annular zone of an annular detector disposed between an electron source and the objective lens and the secondary electrons at low energy can be detected in an outer annular zone. Since the outer annular zone is quartered in the form of sectors to permit selection of azimuth angles of secondary electron emission, acquisition of a shadow image can be assured.